The present invention relates to exhaust gas purifying systems for engines, which are provided with a catalyst displaying a three-way purification function when at least the exhaust gas is in a state corresponding substantially to a theoretical air/fuel ratio, and in which the engine is ordinarily switched between an operating state in which the air/fuel ratio in the combustion chamber is lean and an operating state in which it is substantially at the theoretical air/fuel ratio or richer than that.
Conventionally, as exhaust gas purifying systems for this kind of engine, systems are known, in which a so-called three-way catalyst is arranged in the exhaust duct of the engine, and an NOx absorbing/reducing catalyst including an NOx absorbing material is arranged nearby on the downstream side, as disclosed for example in Japanese Laid-Open Patent Publication (Tokkai) No. 11-200853. For the NOx absorbing material, it is possible to use, for example, an alkaline earth metal such as barium, and when the air/fuel ratio of the exhaust gas is in a predetermined lean state, that is, in an oxygen-rich atmosphere with an oxygen concentration of, for example, at least 4% in the exhaust gas, the NOx in the exhaust gas is oxidized, and absorbed as nitrate, whereas when the oxygen concentration drops, the absorbed nitrate undergoes a substitution reaction with the CO in the exhaust gas, and NOx is released, while the CO is absorbed as carbonate.
When the air/fuel ratio of the exhaust gas is in the lean state, the NOx in the exhaust gas is absorbed by the above-mentioned NOx absorbing material, and the exhaust gas is purified. Furthermore, when the engine is operated substantially at the theoretical air/fuel ratio, and the air/fuel ratio of the exhaust gas is in a state corresponding substantially to the theoretical air/fuel ratio, that is, when the oxygen concentration is about 0.5 to 1% or lower, almost all of the HC, CO and NOx in the exhaust gas is purged due to the three-way purification function of the catalyst, and the NOx released from the NOx absorbing material reacts with the HC and CO, and reduction purification is performed.
Moreover, the NOx absorbing material has the property that, as the amount of absorbed NOx increases, the ability of the NOx absorbing material to absorb NOx decreases, so that in the above conventional example, when the engine shifts from an operating state with a lean air/fuel ratio to an operating state at substantially the theoretical air/fuel ratio, the air/fuel ratio is enriched considerably for a short time, that is, in spike-form, thus promoting the release of NOx.
Like the NOx absorbing/reducing catalyst, the so-called three-way catalyst, which displays a three-way purification function when the air/fuel ratio of the exhaust gas is in a state corresponding substantially to the theoretical air/fuel ratio, generally includes an oxygen absorbing material, such as ceria (CeO2). This oxygen absorbing material has the property that it absorbs oxygen in the exhaust gas when the oxygen concentration is larger than a certain level (for example 0.5%), and releases oxygen when the oxygen concentration is lower than that value, so that the variations of the oxygen concentration in the exhaust gas are dampened, and the three-way purification function can be increased.
However, when a three-way catalyst and an NOx absorbing/reducing catalyst are arranged in series in the exhaust duct of the engine, as in the above conventional example, and the air/fuel ratio of the exhaust gas is changed from a lean state to a state corresponding substantially to the theoretical air/fuel ratio, then oxygen is released from the oxygen absorbing material included in the catalysts, so that the air/fuel ratio shifts locally toward the lean side in the vicinity of the two catalysts, and the HC and CO in the exhaust gas are used up by reacting with the oxygen, and eventually it becomes impossible to release NOx efficiently from the NOx absorbing/reducing catalyst located downstream.
Regarding this aspect, the above conventional example estimates the amount of oxygen released by the three-way catalyst by considering the operation history of the engine and the oxygen absorbing capability in the three-way catalyst, anticipates that HC or CO are consumed by reacting with the oxygen, and enriches the air/fuel ratio in the combustion chamber considerably so that a sufficient amount of HC and Co are supplied to the downstream NOx absorbing/reducing catalyst.
However, in the above-described conventional exhaust gas purifying system, when the air/fuel ratio in the combustion chamber of the engine is switched from the lean state to the state of substantially the theoretical air/fuel ratio, the amount of injected fuel increases spike-like for a short time, enriching the air/fuel ratio considerably, so that as described above, even when increasing the HC and CO in the exhaust gas to match the amount of oxygen released by the catalyst, in actuality, a lot of the HC and CO are blown through the catalyst and are emitted into the atmosphere.
Thus, the HC and CO emitted into the atmosphere increases sharply, even if only temporarily, and the condition of the exhaust gas deteriorates. Moreover, the oxygen released by the catalysts may not react sufficiently with the HC and CO, whereby in the vicinity of the catalysts, the air/fuel ratio of the exhaust gas shifts to the leaner side, leaving the appropriate air/fuel ratio range for displaying the three-way purification function, and there is the risk that the catalysts"" property of purifying the exhaust gas drops.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an exhaust gas purifying system for an engine, in which a catalyst having a three-way purification function is provided in an exhaust duct, and in which the engine is usually switched between an operating state of lean air/fuel ratio and an operating state with an air/fuel ratio of substantially the theoretical air/fuel ratio or richer than that, wherein a temporary deterioration of the state of the exhaust gas caused by emission of oxygen from the catalyst can be prevented, while suppressing deterioration of the total fuel efficiency when considered for all operating regions.
In order to attain these objects, when the engine has shifted from an operating state with lean air/fuel ratio to an operating state with richer air/fuel ratio, the control target value of the air/fuel ratio is corrected to be slightly richer than the theoretical air/fuel ratio, so as to match the resulting enrichment of the air/fuel ratio with the pace in which oxygen is released from the catalyst.
More specifically, in accordance with a first aspect of the present invention, an exhaust gas purifying system for an engine includes a catalyst displaying a three-way purification function when an air/fuel ratio state of at least an exhaust gas corresponds substantially to a theoretical air/fuel ratio; a first air/fuel ratio control means for controlling an air/fuel ratio in a combustion chamber of the engine by switching to either a state that is leaner than the theoretical air/fuel ratio or a state that is richer than that, depending on the operating state of the engine; and an air/fuel ratio correction means for correcting the air/fuel ratio in the combustion chamber temporarily to a state that is even richer, when the first air/fuel ratio control means switches the air/fuel ratio in the combustion chamber from a lean state to a richer state. When the air/fuel ratio in the combustion chamber has been switched from a lean state to a richer state, the air/fuel ratio correction means sets the air/fuel ratio in the combustion chamber to a state that is slightly richer than the theoretical air/fuel ratio, so as to match a release of oxygen from the catalyst.
With this configuration, first, when the engine is in an operating state with lean air/fuel ratio, the exhaust gas from the engine is in a state of high oxygen concentration, so that a portion of the oxygen in the exhaust gas is gradually absorbed by the catalyst. Then, when the operating state of the engine changes and the air/fuel ratio in the combustion chamber is switched by the first air/fuel ratio control means to a state in which the air/fuel ratio is richer than in the lean state, the air/fuel ratio in the combustion chamber is temporarily corrected to an even richer state by the air/fuel ratio correction means.
In this situation, the air/fuel ratio in the combustion chamber is corrected by the air/fuel ratio correction means to a state that is slightly richer than the theoretical air/fuel ratio so as to match the release of oxygen from the catalyst, so that the air/fuel ratio state of the exhaust gas from the combustion chamber of the engine becomes a state of a slightly richer (lower) oxygen concentration than in the state corresponding to the theoretical air/fuel ratio. This cancels the influence of the oxygen release from the catalyst, thus maintaining the local air/fuel ratio state in the vicinity of the catalyst in a state corresponding to substantially the theoretical air/fuel ratio. Thus, the three-way purification performance of the catalyst can be maintained reliably. Moreover, there is no spike-like enrichment of the air/fuel ratio state of the exhaust gas as in the related art (JP H11-200853), so that HC and CO in the exhaust gas are not blown through the catalyst.
The exhaust gas purifying system for an engine according to a second aspect of the present invention further includes a detection means for detecting concentration of a predetermined component of the exhaust gas upstream from the catalyst. Furthermore, the first air/fuel ratio control means controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio when the engine is in a predetermined operating region of low revolution speeds and low loads, and feedback controls the air/fuel ratio in the combustion chamber to a target value A/F near the theoretical air/fuel ratio, based on a signal from the detection means, when the engine is in a preset operating region of higher revolution speeds and/or higher loads than that predetermined operating region. Additionally, when the engine has shifted from the predetermined operating region to the preset operating region, the air/fuel ratio correction means sets the target value A/F of the air/fuel ratio feedback control with the first air/fuel ratio control means within a range of 14.0xe2x89xa6A/F less than 14.7, until a predetermined air/fuel ratio correction period has passed.
With this aspect of the present invention, first, the air/fuel ratio control means controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio when the engine is in a predetermined operating region of low revolution speeds and low loads. In this situation, the exhaust gas is in a state of high oxygen concentration, so that a portion of the oxygen in the exhaust gas is gradually absorbed by the catalyst. Then, when the engine shifts from the predetermined operating region to the preset operating region, the air/fuel ratio correction means sets the target value A/F of the air/fuel ratio feedback control with the first air/fuel ratio control means within a range of at least 14.0 and less than 14.7, that is, slightly richer than the theoretical air/fuel ratio, until a predetermined air/fuel ratio correction period has passed.
Thus, the air/fuel ratio state of the exhaust gas becomes slightly richer than in the state corresponding to the theoretical air/fuel ratio, that is, a state in which the oxygen concentration is low, and this drop of the oxygen concentration cancels the influence of the oxygen release by the catalyst, so that the local air/fuel ratio state in the vicinity of the catalyst is maintained in a state corresponding substantially to the theoretical air/fuel ratio. Moreover, the state of the air/fuel ratio of the exhaust gas is not enriched spike-like, so that the HC and CO in the exhaust gas are not blown through the catalyst.
That is to say, the oxygen released from the catalyst is reacted in just the right amount with the HC and CO in the exhaust gas, preventing a deterioration of the state of the exhaust gas due to blowing through of HC or CO, and maintaining a stable three-way purification performance of the catalyst.
The exhaust gas purifying system for an engine according to a third aspect of the present invention further includes a detection means for detecting concentration of a predetermined component of the exhaust gas upstream from the catalyst. Furthermore, the first air/fuel ratio control means basically controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio when the engine is in a predetermined operating region of low revolution speeds and low loads, and feedback controls the air/fuel ratio in the combustion chamber to a target value A/F near the theoretical air/fuel ratio, based on a signal from the detection means, when the engine has entered a preset specified operating state in the predetermined operating region. Moreover, when the engine has entered the specified operating state in the predetermined operating region, the air/fuel ratio correction means sets the target value A/F of the air/fuel ratio feedback control with the air/fuel ratio control means within a range of 14.0xe2x89xa6A/F less than 14.7, until a predetermined air/fuel ratio correction period has passed.
Thus, when the engine has entered a specified operating state in the predetermined operating region with low revolution speeds and low loads, that is, when for example the engine has shifted from a normal operating state to an accelerating operating state, a similar operational effect as in the second aspect of the present invention can be attained.
The exhaust gas purifying system for an engine according to a fourth aspect of the present invention further includes an oxygen absorption amount estimation means for estimating the amount of oxygen absorbed by catalyst when the engine is in the predetermined operating region, and a correction period setting means for setting a length of the air/fuel ratio correction period in accordance with a value estimated by the oxygen absorption amount estimation means.
Thus, the length of the air/fuel ratio correction period is modified by the correction period setting means in accordance with the amount of oxygen absorbed by the catalyst, so that, for example, when the engine has shifted from the predetermined operating region to the preset operating region, the air/fuel ratio state of the exhaust gas is suitably enriched continuously while oxygen is being released from the catalyst, thereby sufficiently attaining the operational effects of the first to third aspects of the present invention.
According to the fifth aspect of the present invention, the exhaust gas purifying system for an engine according to the fourth aspect of the present invention further includes a correction prohibiting means for prohibiting corrective control of the air/fuel ratio with the air/fuel ratio correction means when the value estimated by the oxygen absorption amount estimation means is not greater than a predetermined value.
Thus, when the amount of oxygen absorbed by the catalyst is not greater than a predetermined value and very small, the amount of oxygen released by the catalyst is very small too, so that in this situation, its adverse effect is very small, and improvement of the fuel efficiency can be better attained by not performing a correction of the air/fuel ratio to the richer side.
In the exhaust gas purifying system for an engine according to a sixth aspect of the present invention, the catalyst is an NOx catalyst including an NOx absorbing material that absorbs NOx in the exhaust gas in an oxygen-rich atmosphere, and releases absorbed NOx when the oxygen concentration has dropped.
Thus, when the engine is in a predetermined operating region, and the air/fuel ratio in the combustion chamber is in a state that is leaner than the theoretical air/fuel ratio, the NOx in the exhaust gas of the oxygen-rich atmosphere is absorbed by the NOx absorbing material, and the exhaust gas is purified. Furthermore, if the engine is in a preset operating region, and the air/fuel ratio in the combustion chamber is substantially the theoretical air/fuel ratio, then the NOx is released by the NOx absorbing material, but this NOx reacts with the HC and the CO in the exhaust gas, and is reduction purified.
According to the seventh aspect of the present invention, in the exhaust gas purifying system for an engine according to a sixth aspect of the present invention, a three-way catalyst is arranged in an exhaust duct upstream from the NOx catalyst, an oxygen concentration sensor for detecting oxygen concentration in the exhaust gas is arranged in the exhaust duct between the two catalysts, and the air/fuel ratio correction period is the period until the oxygen concentration detected by the oxygen concentration sensor has dropped to a preset value or lower.
That is to say, if a three-way catalyst and an NOx catalyst are arranged in order from the upstream side in the exhaust duct of the engine, then, when the air/fuel ratio in the combustion chamber of the engine shifts from a lean state to substantially the theoretical air/fuel ratio, there is the risk that the release and reduction purification of the NOx from the NOx absorbing material in the downstream NOx catalyst is impeded by the release of oxygen from the upstream three-way catalyst.
With the present invention, on the other hand, an oxygen concentration sensor is arranged downstream from the three-way catalyst, and until it is detected by the output of this oxygen concentration sensor that the oxygen concentration in the exhaust gas has dropped to or below a predetermined value, the correction to the rich side of the air/fuel ratio is continued. Thus, while oxygen is actually released from the three-way catalyst on the upstream side in the exhaust duct, the air/fuel ratio in the combustion chamber is enriched accordingly, and when no more oxygen is released from the three-way catalyst, the enriching correction of the air/fuel ratio is terminated. Thus, the adverse effect due to the oxygen release from the three-way catalyst is eliminated, and the release and reduction purification of NOx from the NOx absorbing material in the downstream NOx catalyst is suitably promoted.
In the exhaust gas purifying system for an engine according to an eighth aspect of the present invention, the catalyst is a three-way catalyst, the exhaust gas purifying system further includes a detection means for detecting concentration of a predetermined component of the exhaust gas upstream from the three-way catalyst, and an NOx catalyst having an NOx absorbing material that absorbs NOx in the exhaust gas in an oxygen-rich atmosphere and releases absorbed NOx when the oxygen concentration drops is arranged in an exhaust duct downstream from the three-way catalyst. Furthermore, an oxygen concentration sensor for detecting oxygen concentration in the exhaust gas is arranged in the exhaust duct between the three-way catalyst and the NOx catalyst. The first air/fuel ratio control means controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio when the engine is in a predetermined operating region of low revolution speeds and low loads, and controls the air/fuel ratio in the combustion chamber to substantially the theoretical air/fuel ratio, when the engine is in a preset operating region of higher revolution speeds and/or higher loads than that predetermined operating region. When the engine has shifted from the predetermined operating region to the preset operating region, the air/fuel ratio correction means corrects the target value of the air/fuel ratio control with the first air/fuel ratio control means to be richer than the theoretical air/fuel ratio, based on a signal from the oxygen concentration sensor, until a predetermined air/fuel ratio correction period has passed.
Thus, as in the second aspect of the present invention, when the engine has shifted to a preset region from a predetermined operating region with low revolution speeds and low loads, the air/fuel ratio correction means corrects the air/fuel ratio in the combustion chamber to a state that is slightly richer than the theoretical air/fuel ratio, canceling the release of oxygen from the three-way catalyst on the upstream side of the exhaust duct, so that the air/fuel ratio state of the exhaust gas in the vicinity of the three-way catalyst and the NOx catalyst is maintained at a suitable state that is obtained by the three-way purification function, and the release and reduction purification of the NOx from the NOx absorbing material in the NOx catalyst is suitably promoted, while preventing a deterioration of the exhaust gas state.
Moreover, in this situation, the oxygen concentration sensor is arranged between the three-way catalyst and the NOx catalyst, and based on the output from this oxygen concentration sensor, the air/fuel ratio correction means corrects the air/fuel ratio towards the richer side, so that the air/fuel ratio is suitably corrected in accordance with the release condition of the oxygen from the three-way catalyst.
In the exhaust gas purifying system for an engine according to a ninth aspect of the present invention, the catalyst is a three-way catalyst, the exhaust gas purifying system further including a detection means for detecting concentration of a predetermined component of the exhaust gas upstream from the three-way catalyst, and an NOx catalyst having an NOx absorbing material that absorbs NOx in the exhaust gas in an oxygen-rich atmosphere, and releases absorbed NOx when the oxygen concentration drops is arranged in an exhaust duct downstream from the three-way catalyst. Furthermore, an oxygen concentration sensor for detecting oxygen concentration in the exhaust gas is arranged in the exhaust duct between the three-way catalyst and the NOx catalyst. Also, the first air/fuel ratio control means basically controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio when the engine is in a predetermined operating region of low revolution speeds and low loads, and switches the air/fuel ratio in the combustion chamber to substantially the theoretical air/fuel ratio, when the engine enters a preset specified operating state in the predetermined operating region. When the engine enters the specified operating state in the predetermined operating region, the air/fuel ratio correction means corrects the target value of the air/fuel ratio control with the first air/fuel ratio control means to be richer than the theoretical air/fuel ratio, based on a signal from the oxygen concentration means, until a predetermined air/fuel ratio correction period has passed.
Thus, when the engine has entered a specified operating state in the predetermined operating region of low revolution speeds and low loads, that is, when for example the engine has shifted from a normal operating state to an accelerating operating state, or when the oxygen concentration in the exhaust gas is intentionally decreased in order to release and reduction purify NOx from the NOx absorbing material of the NOx catalyst, a similar operational effect as in the eighth aspect of the present invention can be attained.
According to the tenth aspect of the present invention, in the exhaust gas purifying system for an engine according to a eighth or ninth aspect of the present invention, the air/fuel ratio correction means is configured so as to terminate the correction control of the air/fuel ratio when the oxygen concentration in the exhaust gas detected by the oxygen concentration sensor has dropped to a preset value or lower.
Thus, while oxygen is actually released from the three-way catalyst, it is possible to reliably continue the correction of the air/fuel ratio accordingly.
Furthermore, the exhaust gas purifying system for an engine according to any of the eleventh to sixteenth aspect of the present invention focuses on the fact that in particular when the engine is installed in a vehicle, under regular operating conditions, the frequency in which the engine is in a rich operating state near the theoretical air/fuel ratio is relatively low, and in this regular condition, the target value of the air/fuel ratio near the theoretical air/fuel ratio is set to a value that is relatively richer than outside the regular condition, that is under a predetermined condition.
That is to say, in the exhaust gas purifying system for an engine according to a eleventh aspect of the present invention, a detection means for detecting concentration of a predetermined component of the exhaust gas is arranged in an exhaust duct upstream from the three-way catalyst, the first air/fuel ratio control means controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio when the engine is in a predetermined operating region of low revolution speeds and low loads, and feedback controls the air/fuel ratio in the combustion chamber to a target value near the theoretical air/fuel ratio, based on a signal from the detection means, when the engine is in a preset operating region of higher revolution speeds and/or higher loads than that predetermined operating region, furthermore the exhaust gas purifying system is further provided with a second air/fuel ratio control means, which prohibits control with the first air/fuel ratio control means under a predetermined condition and when the engine is in the predetermined operating region or the preset operating region, and feedback-controls the air/fuel ratio in the combustion chamber to a target value near the theoretical air/fuel ratio, based on the signal from the detection means. The target value for feedback control of the air/fuel ratio with the first air/fuel ratio control means is set to a value that is richer than the control target value of the second air/fuel ratio control means.
Thus, under regular conditions, when the engine is in the predetermined operating region of low revolution speeds and low loads, the first air/fuel ratio control means controls the air/fuel ratio in the combustion chamber to a state that is leaner than the theoretical air/fuel ratio.
In this situation, the air/fuel ratio state of the exhaust gas is in a state of high oxygen concentration, so that the oxygen in the exhaust gas is absorbed by the catalyst.
Then, when the engine shifts to a preset operating region, the first air/fuel ratio control means feedback controls the air/fuel ratio in the combustion chamber to a target value near the theoretical air/fuel ratio. In this situation, also the air/fuel ratio state of the exhaust gas corresponds to substantially the theoretical air/fuel ratio, so that oxygen is released from the catalyst, but the target value of the air/fuel ratio feedback control is set to be accordingly richer, so that the air/fuel ratio state near the catalyst does not deviate considerably to the leaner side, and consequently, the three-way purification function of the catalyst is not harmed. Furthermore, the air/fuel ratio of the exhaust gas is not enriched spike-like, so that the HC and CO in the exhaust gas are not blown through the catalyst.
On the other hand, under a predetermined condition, the air/fuel ratio in the combustion chamber of the engine is controlled by the second air/fuel ratio control means to substantially the theoretical air/fuel ratio or a state that is richer than that, so that in this situation, no oxygen is absorbed by the catalyst. Therefore, under this predetermined condition, when the air/fuel ratio in the combustion chamber is feedback controlled to a target value near the theoretical air/fuel ratio, the control target value can be set to a relatively leaner side, thus achieving a decrease of the fuel consumption.
That is to say, this aspect of the present invention takes advantage of the fact that under normal conditions, the engine is mostly in an operating state with lean air/fuel ratio, and rarely in an operating state at the theoretical air/fuel ratio or richer than that, and the control target value for feedback controlling the air/fuel ratio to near the theoretical air/fuel ratio is set to be slightly richer than under the predetermined condition when oxygen accumulates in the catalyst, so that the air/fuel ratio is not enriched spike-like, and the oxygen released from the catalyst can be caused to react in just the right amount with the HC and the CO in the exhaust gas. Thus, it can be prevented that a portion of the HC and CO in the exhaust gas is blown through the catalyst, and the purification performance of the catalyst can be stabilized. Furthermore, since the engine is not frequently operated near the theoretical air/fuel ratio during normal conditions, the deterioration of the total fuel consumption when regarded over the entire operating region is small, even when the air/fuel ratio at that time is set on the rich side.
According to the twelfth aspect of the present invention, in the exhaust gas purifying system for an engine according to the eleventh aspect of the present invention, the predetermined condition is that the engine is in a not-yet warmed-up state.
Thus, in the not-yet warmed-up state, in which gasification/atomization of the fuel is difficult, the air/fuel ratio in the combustion chamber is controlled by the second air/fuel ratio control means to be substantially the theoretical air/fuel ratio or in a state richer than that, regardless of the operating state of the engine, thereby ensuring combustion stability of the engine.
According to the thirteenth aspect of the present invention, the exhaust gas purifying system for an engine according to the twelfth aspect of the present invention further includes a catalyst temperature state determination means for determining whether a temperature of the catalyst is lower than a preset temperature, and a target value correction means for correcting the target value of the air/fuel ratio feedback control with the second air/fuel ratio control means to a value that is leaner than the theoretical air/fuel ratio, when the catalyst temperature state determination means has determined that the catalyst is in a low-temperature state.
Thus, even when the engine is in the not-yet warmed-up state, when the catalyst is in a not-yet activated low temperature state, the target value of the air/fuel ratio feedback control is set to be leaner than the theoretical air/fuel ratio, and by reacting the oxygen in the exhaust gas with the unburned fuel, the heating of the catalyst can be promoted. It should be noted that even when the air/fuel ratio is slightly leaner than the theoretical air/fuel ratio, the combustion stability of the engine is not harmed.
According to the fourteenth aspect of the present invention, the exhaust gas purifying system for an engine according to the eleventh aspect of the present invention is further provided with a defect state determination means for determining when at least one of the first air/fuel ratio control means and the sensors and actuators that are necessary for executing control with the first air/fuel ratio control means is defective, and the predetermined condition is that a defect has been determined by the defect state determination means.
That is to say, when the control with the first air/fuel ratio control means cannot be performed properly, the control with the first air/fuel ratio is prohibited, and the control of the air/fuel ratio is carried out with the second air/fuel ratio control means.
According to the fifteenth aspect of the present invention, in the exhaust gas purifying system for an engine according to the twelfth or fourteenth aspect of the present invention, the target value of air/fuel ratio feedback control with the second air/fuel ratio control means is substantially the theoretical air/fuel ratio.
Thus, even when the engine is in the not-yet warmed-up state or a sensor or the like is defective, the catalyst displays the three-way purification function, and a high exhaust gas purification performance can be stably ensured, while maintaining the combustion stability of the engine.
According to the sixteenth aspect of the present invention, in the exhaust gas purifying system for an engine according to the eleventh aspect of the present invention, the catalyst is a three-way catalyst, and an NOx catalyst, which includes NOx absorbing material that absorbs NOx in the exhaust gas in an oxygen-rich atmosphere, and releases absorbed NOx when the oxygen concentration drops, is provided downstream from the three-way catalyst.
With this configuration, the engine is ordinarily in the predetermined operating region with low revolution speeds and low loads, and when the air/fuel ratio in the combustion chamber is controlled to a state that is leaner than the theoretical air/fuel ratio, the air/fuel ratio state of the exhaust gas becomes a state of high oxygen concentration, and in this situation, due to the three-way catalyst on the upstream side and the NOx catalyst on the downstream side, the HC and CO in the exhaust gas are oxidized and purged, and the NOx is absorbed and eliminated. Moreover, the oxygen in the exhaust gas is absorbed by these two catalysts.
Subsequently, when the engine shifts to the preset operating region, and the air/fuel ratio in the combustion chamber is feedback controlled to a target value near the theoretical air/fuel ratio, the air/fuel ratio state of the exhaust gas becomes a state corresponding substantially to the theoretical air/fuel ratio, and in this situation, the two catalysts display a three-way purification function, and the HC, CO and NOx in the exhaust gas are purged. Then, in this situation, oxygen is released by the two catalysts, but since the target value of the air/fuel ratio feedback control has been set to a value that is accordingly richer, the air/fuel ratio state in the vicinity of the catalysts does not deviate very much, and consequently, the three-way purification function of the catalysts is not harmed. Furthermore, a suitable amount of HC and CO is supplied also to the downstream NOx catalyst, so that the release and reduction purification of the NOx from the NOx catalyst is sufficiently promote.
As explained above, with the exhaust gas purifying system for an engine according to the first aspect of the present invention, a catalyst is provided that displays a three-way purification function at least when the air/fuel ratio state of the exhaust gas corresponds substantially to the theoretical air/fuel ratio, and if the engine is switched between an operating state with lean air/fuel ratio and an operating state with an air/fuel ratio that is richer than that, when the engine has shifted from an operating state with lean air/fuel ratio to an operating state with a richer air/fuel ratio, the adverse effect due to release of oxygen from the catalyst is cancelled by correcting the target value for control of the air/fuel ratio temporarily to a slightly richer state, so that the local air/fuel ratio state in the vicinity of the catalyst can be maintained at a state corresponding substantially to the theoretical air/fuel ratio. Thus, the three-way purification function of the catalyst can be maintained stable while preventing the HC and CO in the exhaust gas from blowing through and deteriorating the exhaust gas state.
With the exhaust gas purifying system for an engine in accordance with the second or third aspect of the present invention, if the engine is switched between an operating state with lean air/fuel ratio and an operating state of substantially the theoretical air/fuel ratio, when the engine is shifted from an operating state with a lean air/fuel ratio to an operating state of substantially the theoretical air/fuel ratio, the feedback control of the air/fuel ratio begins, and the target value A/F of this feedback control is corrected to 14.0 A/F less than 14.7, thus reliably attaining the effects of the first aspect of the present invention.
According to the fourth aspect of the present invention, the period in which the air/fuel ratio is corrected to the richer side is modified in accordance with the amount of oxygen absorbed by the catalyst, so that the air/fuel ratio state of the exhaust gas can be appropriately enriched continuously while oxygen is released from the catalyst, and the effects of the first to third aspects of the invention can be sufficiently attained.
According to the fifth aspect of the present invention, an increase in fuel efficiency is attained by not performing the correction of the air/fuel ratio when the amount of oxygen absorbed by the catalyst is very small.
According to the sixth aspect of the present invention, the NOx in the exhaust gas is absorbed by the catalyst including an NOx absorbing material, even when the air/fuel ratio in the combustion chamber is in a state that is leaner than the theoretical air/fuel ratio, so that the exhaust gas can be purified.
According to the seventh aspect of the present invention, a three-way catalyst and a catalyst including an NOx absorbing material are arranged in that order from the upstream side in the exhaust duct of an engine, an oxygen concentration sensor is arranged between them, and when, based on the signal from this oxygen concentration sensor, the oxygen concentration downstream from the three-way catalyst has actually dropped, the enriching correction of the air/fuel ratio is terminated, so that the air/fuel ratio state of the exhaust gas can be suitably enriched continuously until actually no more oxygen is released from the three-way catalyst, thereby making it possible to appropriately promote the release and reduction purification of the NOx in the downstream catalyst.
With the exhaust gas purifying system according to the eighth or ninth aspect of the present invention, if a three-way catalyst and an NOx catalyst are arranged in that order from the upstream side in the exhaust duct of an engine, when the engine has shifted from an operating state with a lean air/fuel ratio to an operating state at substantially the theoretical air/fuel ratio, a sufficient exhaust gas purification performance can be attained with the two catalysts while preventing a deterioration of the exhaust gas state caused by the release of oxygen from the two catalysts, as in the second and third aspects of the present invention.
According to the tenth aspect of the present invention, in addition to the effects of the eighth and ninth aspects of the present invention, the air/fuel ratio state of the exhaust gas can be suitably enriched continuously until actually no more oxygen is released from the three-way catalyst, so that the release and reduction purification of the NOx with the NOx catalyst can be adequately promoted, as in the seventh aspect of the present invention.
Furthermore, the exhaust gas purifying system for an engine according to the eleventh aspect of the present invention includes a catalyst having a three-way purification function near the theoretical air/fuel ratio, and under normal conditions, the engine is switched between an operating state with lean air/fuel ratio and an operating state with substantially the theoretical air/fuel ratio or richer than that, whereas if under a predetermined condition, the operation with the lean air/fuel ratio is not performed, then the control target value of the air/fuel ratio under normal conditions when the engine is operated near the theoretical air/fuel ratio is set to a value that is relatively richer than under the predetermined conditions, so that the oxygen released by the catalyst in this situation is reacted in just the right amount with the HC and CO in the exhaust gas, and a temporary deterioration of the exhaust gas state caused by the release of oxygen from the catalyst can be prevented, while suppressing a deterioration of the total fuel consumption considering all operating regions.
According to the twelfth aspect of the present invention, in a not-yet warmed-up state, the air/fuel ratio in the combustion chamber is controlled to a state of substantially the theoretical air/fuel ratio or richer than that, regardless of the operating state of the engine, thus ensuring combustion stability of the engine.
According to the thirteenth aspect of the present invention, when the catalyst is not yet activated, the target value of the air/fuel ratio feedback control is set to leaner than the theoretical air/fuel ratio, thus promoting the heating of the catalyst.
According to the fourteenth aspect of the present invention, a suitable air/fuel ratio control can be performed with the second air/fuel ratio control means, even when a sensor, actuator or the like is defective, and the control cannot be performed properly with the first air/fuel ratio control means.
According to the fifteenth aspect of the present invention, a high exhaust gas purification performance of the catalyst can be reliably ensured while maintaining combustion stability of the engine, even if the engine is in a not-yet warmed-up state, or a sensor or the like is defective.
According to the sixteenth aspect of the present invention, by arranging a three-way catalyst in the exhaust duct of the engine, and arranging an NOx catalyst downstream therefrom, a high exhaust gas purification performance can be attained, regardless of the air/fuel ratio state of the exhaust gas. On the other hand, in this case, when the engine is switched from an operating state with lean air/fuel ratio to an operating state near the theoretical air/fuel ratio, the oxygen released by the upstream three-way catalyst may lead to an adverse effect on the release and reduction purification of NOx from the downstream catalyst, so that it is very advantageous that the target value of the air/fuel ratio feedback control near the theoretical air/fuel ratio is set accordingly richer, like in the first aspect of the present invention, thus canceling the influence of the oxygen release.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.